World's Roundest Object!
Veritasium6 HN recommenders · 2013–2018
38.5M views · explainer · dated · Uploaded 2013-03-25
Veritasium explains the effort to redefine the kilogram, which was still defined by a physical artifact, and how a near-perfect silicon sphere from the Avogadro project helps. It also covers the history of the unit's odd naming.
Recommended in kilogram-redefinition threads as a clear, short explainer of the Avogadro and Planck approaches.
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9 comments from Hacker News
"Weirdly, it’s not even known if the IPK is getting lighter, or if the national prototypes are getting heavier — but either way, something is causing these kilos to change weight, by around 50 micrograms every 100 years." Well they COULD always do that thing with the water where they heat it to 4 degrees celcius and measure its volume and weight. Then they'd know which it is. That said, there's a nice video on this: [VIDEO]
For anyone interested in the Planck and Avogadro projects to redefine the kilo, this video is an imperative watch: [VIDEO]
Check out this veritasium video [VIDEO] - It goes into a little detail on how the kilogram got it's name, it is also very interesting in it's own right.
The scientific community is working on the replacement: Known as the Avogadro Project, the plan is to bring together enough atoms of one substance – silicon – to make a kilo. Attention has focused on silicon because: - its characteristics are very well understood - a single crystal of the right size can be grown - its atomic structure is extremely uniform - its widespread use in the computer industry means it can be obtained with relative ease at high purity and resonable cost. A spherical shape was chosen because a sphere has no edges that might get damaged and only one dimension has to be measured in order to calculate its volume. --- [LINK] [VIDEO]
Fantastic video by Vertiasium on this: [VIDEO] (side note: that video is about a sphere, but the current standard kilogram and its copies are actually shaped like little bells)
I stumbled upon a very interesting short documentary about the metric system and the world's roundest object. Well worth 10 minutes of your break today. [VIDEO]
One cubic centimetre of water at 4 degrees Celsius used to define one gram (or more precisely one litre -> one kilogram, which comes out to be the exact same thing) [1], [2]. Although 1 cm^3 of water is still pretty damn close to 1g. [1] [LINK] [2] [VIDEO] (around 1:50, and actually the whole video is excellent)
As Veritasium explains [0], all of the reference kilograms have drifted, and some appear to weigh less than they used to. So even if we know the precise mechanism of action of the drift, it doesn't help with the fact that our measurements are less reliable than they used to be. Of course, what's probably happened is that our measuring tools have gotten more accurate! [0] [VIDEO]
Mole was just redefined as exactly 6.02214076e23 — the number of Si-28 atoms in a perfect 1 kg sphere. This was an intentionally check against the kibble balance measurement. More info from an older Veritasium video: [VIDEO]